linux-kernel - Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Message-ID: <20220331143310.vhsnywx3z7obdycf@ava.usersys.com>
Date:   Thu, 31 Mar 2022 15:33:10 +0100
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Aaron Tomlin <atomlin@...mlin.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Christoph Lameter <cl@...ux.com>, tglx@...utronix.de,
        mingo@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Phil Auld <pauld@...hat.com>
Subject: Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the
 idle tick was stopped too

On Thu 2022-02-17 17:32 +0100, Frederic Weisbecker wrote:
> Then I can see two other issues:
> 
> 1) Can an interrupt in idle modify the vmstat and thus trigger the need to
>    flush it? I believe it's the case and then the problem goes beyond nohz_full
>    because if the idle interrupt fired while the tick is stopped and didn't set
>    TIF_RESCHED, we go back to sleep without calling quiet_vmstat().
> 
> 2) What if we are running task A in kernel mode while the tick is stopped
>    (nohz_full). Task A modifies the vmstat and goes to userspace for a long
>    while.
> 
> Your patch fixes case 1) but not case 2). The problem is that TIMER_DEFERRABLE
> should really be about dynticks-idle only and not dynticks-full. I've always
> been afraid about enforcing that rule though because that would break old
> noise-free setups. But perhaps I should...

Hi Frederic,


Firstly, apologies for the delay.

In reference to case 2:

If I understand correctly, even if TIMER_DEFERRABLE is removed
refresh_cpu_vm_stats() cannot be invoked since the scheduling-clock tick is
disabled i.e. non-deferrable timers are serviced by the tick, no?
So, the only option would be to interrupt the workload - not desirable - or
detect any remaining differentials prior to entering userspace?


Kind regards,

-- 
Aaron Tomlin